I've never worked with a flash before, and have been having some trouble. I have a Nikon D5000 and am using a Sunpack auto 433D. When I take a photo (just to test the exposure) every single on comes out with a horizontal line down the middle, as if to split the photo in half, where the top half is affected by the flash and the bottom is not. Anybody know what I'm doing wrong/can help me out? Thanks so much!

It could be several things but my guess you have a shutter setting faster than your camera instructions recommend. This means the shutter is to quick to get the entire flash in EI
one half OK the other no flash. This only happens on a focal plane shutter which you have. Most modern camera's the maximum speed is usually about 125 of a second you could try a lower shutter speed. On saying this most modern focal blade shutters work from top to bottom so what I,ve said would not follow. I do not know your camera so you need to check if your shutter goes from side to side or to to bottom. If your shutter goes top to bottom,and you have one side of your picture flashed and the other non flashed. I would guess you may have a fault in your camera.

One of the shutter curtains is still covering part of the image sensor when the flash fires.

Your D5000 comes with the default set to sync the flash unit when the front shutter curtain is fully open. In the menus you can have the flash sync with the moment when the rear shutter curtain starts to close. (See page 71 of your D5000 user's manual).

The flash x-sync speed is the fastest shutter speed at which both shutter curtains are fully open. At faster shutter speeds one or both shutter curtains cover a portion of the image sensor. In other words the 2 shutter curtains form a slit that moves across the image sensor. The faster the shutter speed, the narrower the slit is.

The lens projects the scene onto the image sensor upside down. So if only the top portion of the photo is obscured by a shutter curtain, it has to be the front curtain and the flash is set to sync with the instant the rear shutter curtains starts to close.

If only the bottom portion of the photo is obscured by a shutter curtain, it has to be the rear shutter curtain and the flash is synced to fire at the instant the front shutter curtain is fully open. This is what you are seeing in your photos.

It really helps if you understand how your camera works.

The D5000 cannot do it, but the more expensive Nikon's and Nikon flash units can do what is known as FP sync mode.

In FP sync mode the flash unit fires several times during the making of an exposure in time with the slit the 2 shutter curtains make, so the entire image sensor is equally exposed to the strobed light during the exposure.

FP sync uses up flash unit batteries in a hurry, and the flash unit has to fire each time at much less than full power so it can re-cycle fast enough.